1. Field of the Invention
The present invention relates generally to a heat exchanger that may be used in a device for heating to a desired infusion temperature a fluid to be infused to a patient. Although the present invention finds particular utility in heating of fluids which are to be supplied intravenously at relatively low flow rates (e.g., below about 2550 ml/hour for fluids to be heated to an infusion temperature of between 38-42 degrees C. from an input temperature into the device of 10 degrees C., or below about 3600 ml/hour for fluids to be heated to such an infusion temperature from an input temperature of 18 degrees C.), it should be understood that other utilities are also contemplated for the present invention (e.g., including in connection with infusion of other types of fluids at other temperatures and flow rates, and infusion of fluids in ways other than by intravenous infusion).
2. Brief Description of Related Prior Art
Many prior art techniques and devices exist for warming fluids to be infused intravenously into humans and other animals. One such conventional device is disclosed in U.S. Pat. No. 5,245,693 (xe2x80x9cthe ""693 patentxe2x80x9d). The ""693 patent is directed to an intravenous fluid heater that includes a disposable cassette containing a heat exchanger. The preferred embodiment of the heat exchanger disclosed in the ""693 patent includes a passageway-defining inner layer sandwiched between a pair of flexible, metal foil membranes. The inner layer defines an extended, e.g., serpentine, path for fluid to be warmed, and serves to space apart and insulate the metal foil membranes from one another. Inlet and outlet ports to the serpentine fluid path are defined in one of the two foil membranes. Heat generated by heating elements which sandwich the heat exchanger is transferred through the metal foil membranes to the fluid flowing through the serpentine path. The heating elements are designed to be graduated, that is, to generate more heat in the area of the inlet portion of the serpentine path than in the area of its outlet.
Unfortunately, the heating device disclosed in the ""693 patent suffers from several disadvantages. First, the heating device disclosed in the ""693 patent is not wearable by the patient adjacent the fluid infusion situs. This means that the length of tubing required to deliver the heated fluid from the device to the infusion situs may vary depending upon where the device is positioned relative to the patient, but will always be longer than that which would be required if the device were being worn by the patient at or near the infusion situs. This means that in the infusion arrangement disclosed in the ""693 patent, the temperature of the heated fluid exiting the heating device will always drop, prior to being infused into the patient, more than would be the case if the heating device were wearable adjacent the infusion situs. The temperature drop of the heated fluid can be especially pronounced at the aforesaid relatively low fluid flow rates. Unfortunately, a significant proportion of intravenous fluid infusions take place at such low flow rates.
A yet further disadvantage of the ""693 patent""s heating arrangement is that although means are included reducing gas bubble formation in the infusion fluid, such means may not always be sufficient when used alone to adequately reduce or eliminate such bubbles prior to infusion of the fluid into the patient. As will be appreciated by those skilled in the art, if left unchecked, this type of condition can be, at minimum, deleterious to patient well-being, and at most, life-threatening.
Another conventional infusion fluid warming device is disclosed in U.S. Pat. No. 5,254,094 (xe2x80x9cthe ""094 patentxe2x80x9d). In the arrangement disclosed in the ""094 patent, a box which may be attached to a patient""s arm is provided. Two chambers are included in the box, containing a heat exchanger element constructed from a continuous length of stainless steel tubing in the form of two parallel coils which are connected to each other by a straight length of tubing. The box includes a passage between the chambers such that a warming fluid may be introduced through an aperture in the box into one of the chambers, flow into the other chamber, and then exit the warmer through another aperture in the box. The infusion fluid to be warmed is supplied to the coils through a first flexible plastic inlet tube and discharged for infusion into a patient through a second flexible plastic tube. The warming fluid is supplied via fluid supply tubing to the box from a separate fluid source that is not dimensioned or suitable for being worn by the patient, such as a water heater. A temperature sensor located in the infusion fluid path between the box and the infusion situs may be provided for generating signals indicative of the temperature of the infusion fluid for provision to a microprocessor contained in the same unit comprising the water heater. The microprocessor also receives outputs from a water temperature sensor and controls the water heater, based upon the outputs from these sensors and a desired infusion fluid temperature set by the user, so as to maintain the heating water at a temperature for heating the infusion fluid to the desired temperature.
Disadvantageously, use of a warming fluid/infusion fluid type of heat exchanger, and a warming fluid heater that is remote from the heat exchanger and not wearable by the patient, make ""094 patent""s arrangement bulky, and relatively difficult to move and set up for use. Also disadvantageously, as is the case in the ""693 patent, the ""094 patent fails to disclose sufficient means for reducing gas bubbles in infusion fluid prior to infusion of the fluid into the patient.
Other examples of infusion fluid warming prior art are disclosed in U.S. Pat. Nos. 5,381,510, 4,731,072, 3,443,060, 3,475,590, 3,485,245, 3,590,215, 3,614,385, 3,640,283, 3,853,479, 4,038,519, 4,108,146, 4,167,663, 4,293,762, 4,309,592, 4,938,279, 4,847,470, 4,574,876, 3,399,536, 4,962,761, 5,125,069, 4,908,014, 4,906,816, 4,844,074, 4,707,587, 4,759,749, 4,782,212, 4,801,777, 4,680,445, 4,678,460, 4,532,414, 4,464,563, 4,314,143, 4,356,383, and 4,878,537. Unfortunately, the prior art disclosed in each of these patents suffers from the aforesaid and/or other disadvantages and drawbacks.
In accordance with the present invention, a heat exchanger is provided that may be used in an intravenous fluid heater that is dimensioned to be wearable adjacent a patient""s intravenous fluid infusion situs. In one embodiment of the present invention, the heat exchanger includes two flexible walls that contact respective heating elements of the fluid heater when the heat exchanger is used in the heater. The heat exchanger also includes at least one member that is inserted between the flexible walls, and together with the flexible walls, defines a flow path for fluid through the heater when the heat exchanger is used in the heater. Alternatively, the at least one member may be eliminated, and the two walls may be spot welded together at selected locations along the walls in such a way as to define the flow path. When the heat exchanger is used in the heater, the heat exchanger is physically unattached to the heater and is removable from the heater (e.g., after being used in the heater).
The heater and heat exchanger may be dimensioned to be wearable by a patient adjacent a fluid infusion situs of the patient. In this embodiment, the flexible walls and member may be made of plastic (e.g., respective polyester plastic films coated with respective outer coatings of acrylic), and the flexible walls may be bonded to the member.
The fluid flow path through the heater may include a fluid inlet, fluid outlet, and serpentine channel between the inlet and outlet. The flexible walls may be identically-dimensioned sheets that contact respective internal sides of flared portions of the member, and completely cover, from opposite respective sides of the member, the channel.
When the heat exchanger is in use in the heater, the heater is in sealing engagement with the heat exchanger, such that an air and liquid tight seal is formed between the heat exchanger and the heater.
In another embodiment of the present invention, at least one of the flexible walls is porous and hydrophobic, and the number and size of the pores in the wall permit gas to be vented from the fluid in the exchanger through the pores, but prevent flow of liquid and bacteria therethrough. In this alternative embodiment, the other flexible wall may be made of polycarbonate and may be thinner than the porous hydrophobic wall of the exchanger. The porous wall may be made of an expanded polytetrafluoro-ethylene material.
In yet another embodiment of the present invention, the heat exchanger may include first and second fluid outlets, and one fluid inlet. The heat exchanger may be constructed such that when the heat exchanger is impinged upon by a pressurizing and gas purging mechanism, the inlet and the first outlet may become occluded, and fluid and gas in the flow path defined by the heat exchanger may be forced (by pressure forces applied to the heat exchanger by the mechanism) through the second outlet, and thence, out of the heat exchanger.
The pressurizing mechanism may comprise a cam system that may apply both a first force and a second force to the heat exchanger. The first force may cause the inlet and the first outlet of the heat exchanger to become occluded. The second force may cause the fluid and the gas to be forced out of the heat exchanger via the second outlet.
The cam mechanism may comprise a plurality of cams. Each of the cams may be actuated by a respective force applied to the cam by a respective change in length and/or shape of a respective wire or filament made of shape-memory alloy. The wire may undergo such change in length and/or shape when heated (e.g., as a result of application of electricity thereto) to apply the force to the cam. The heat exchanger may be disposed in an external housing, and the cam may be positioned between the housing and the heat exchanger.
The heat exchanger may also include or be connected to a hydrophobic membrane that vents the gas to the ambient environment. The hydrophobic membrane may be in fluid communication with the inlet. A check valve may be employed to prevent return fluid flow into the heat exchanger via the second outlet.
Thus, the heat exchanger of the present invention, when used in the heater, is physical unattached to the heater and is removable from the heater when not in use in the heater. Advantageously, this permits the heat exchanger to be disposable/replaceable, and the remainder of the heater to be reusable.
Also in accordance with the present invention, the entire fluid heating assembly (i.e., including the heat exchanger and heater) may be dimensioned so as to be wearable adjacent the patient""s fluid infusion situs. Additionally, when at least one wall of the heat exchanger is porous and gas permeable, or the heat exchanger is constructed for use with the aforedescribed pressurizing and gas purging mechanism, dissolved gas (e.g., air) in the fluid in the heat exchanger may be vented or purged to the ambient environment prior to infusion of the fluid into the patient. Advantageously, these features provided in accordance with the present invention permit a fluid heater assembly using a heat exchanger in accordance with the present invention to overcome the aforesaid and other disadvantages of the ""693 and ""094 patents.
Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds and upon reference to the Drawings, wherein like numerals depict like parts, and in which: